Spheroidal graphite cast iron is in wide use in recent years as components for vehicles including motor vehicles, machine parts, etc., because the spheroidal graphite cast iron has excellent tensile strength and ductility. In particular, spheroidal graphite cast iron is used in brake calipers which are important as safety components for vehicles such as motor vehicles in order to ensure the quality thereof.
Since there is a desire for weight reduction in these products, spheroidal graphite cast iron also is required to be reduced in thickness. In the case where spheroidal graphite cast iron is applied as a cast metal having a thin-wall part, a cooling rate is increased in the thin-wall part thereof and this results in the formation of a chill phase (abnormal structure). Since this chill phase has an exceedingly hard structure, the machinability is reduced and machining is difficult to be performed especially when a surface layer thereof having an enhanced tendency to chill phase formation has hardened.
Because of this, in the case of using spheroidal graphite cast iron to produce a product having a thin-wall part, the cast molten iron is usually subjected to a spheroidization treatment and further subjected to an inoculation treatment multiple times in order to inhibit chill phase formation. In particular, since the spheroidal graphite cast iron for use in components for motor vehicles is frequently required to be inhibited from having a chill structure and to retain a high level of balance between strength and ductility, various measures are being taken in producing thin-wall spheroidal graphite cast iron.
For example, a spheroidizing agent containing a rare-earth element (rare earth) is used in order to more reliably conduct spheroidization and graphitization. Patent Documents 1 to 3 disclose the spheroidizing agents containing a rare earth in a given amount (in the range of about 0.5 to 9% by mass) and the spheroidal graphite cast iron produced using the spheroidizing agents. Rare earths not only have the effect of accelerating graphite spheroidization on the basis of both a deoxidizing and desulfurizing function and the function of lowering the action of spheroidization-inhibitory elements but also serve, for example, to accelerate graphitization, prevent chill phase formation, inhibit chunky graphite formation, and inhibit fading, on the basis of the effect of yielding graphite nuclei, etc. Hence, rare earths are elements exceedingly profitable for spheroidal graphite cast iron. Especially in the production of thin-wall spheroidal graphite cast iron for use in components for motor vehicles, use of a spheroidizing agent containing such a rare earth is regarded as essential for preventing chill phase formation in the thin-wall part.
However, rare earths are resources which localize in limited regions on earth, and specific countries have exceedingly high shares of the international production thereof. Ninety percents of the demand thereof in Japan also depend on imports from the specific countries. In recent years, rare earths have become indispensable resources not only in the field of cast metal but also in the fields of electronic appliances, magnetic components, glass appliances, catalysts, etc., and the prices thereof are skyrocketing. It is thought that the prices and production amounts thereof fluctuate considerably in the future, depending on the circumstances of the producing countries, and there is a high possibility that both the prices and the supply amounts might become exceedingly unstable.
Consequently, an imminent subject is to establish a method for producing spheroidal graphite cast iron using a spheroidizing agent which has a reduced rare-earth content or contains no rare earth, in order to ensure production amounts and quality of vehicle components using the spheroidal graphite cast iron.
There have hitherto been spheroidizing agents containing no rare earth. For example, Patent Document 4 discloses a spheroidization treatment using an Mg-based spheroidizing agent which contains no rare earth at all, from the standpoint of preventing chunky graphite from crystallizing out when large thick spheroidal graphite cast iron is produced.